As the development of biometric devices for identity verification, and in particular of fingerprint sensing devices, has lead to devices which are made smaller, cheaper and more energy efficient, the possible applications for such devices are increasing.
In particular fingerprint sensing has been adopted more and more in, for example, consumer electronic devices, due to small form factor, relatively beneficial cost/performance factor and high user acceptance (in particular in comparison to retina scanning etc).
Capacitive fingerprint sensing devices, built based on CMOS technology for providing the fingerprint sensing elements and auxiliary logic circuitry, are increasingly popular as such sensing devices can be made both small and energy efficient while being able to identify a fingerprint with high accuracy. Thereby, capacitive fingerprint sensors are advantageously used for consumer electronics, such as portable computers, tablets and mobile phones.
A fingerprint sensing chip typically comprises an array of capacitive sensing elements providing a measure indicative of the capacitance between several sensing structures and a finger placed on the surface of the fingerprint sensor. The sensing chip may further comprise logic circuitry for handling addressing of the array of sensing elements.
Furthermore, the sensing chip is often mounted on a separate readout substrate comprising readout circuitry, wherein contact pads of the sensing chip are provided for enabling an electrical connection via means of wire-bonding to corresponding contact pads of the readout substrate. The readout substrate may for example be a printed circuit board (PCB).
However, a wire bond protrudes above the surface of the sensing chip with a distance corresponding to the height of the bond plus the curvature of the bond-wire, commonly referred to as the wire bond loop height.
Accordingly, protruding wire bonds introduce constraints in the assembly and design of a fingerprint sensor. In particular, in many applications is it desirable to provide a fingerprint sensing device which is flat, both for aesthetic reasons and as elevated portions of a sensing surface may lead to that a finger is partially lifted near the protruding portion. In addition, any protruding portions of an electrically conductive feature, relative to the pixels, such as e.g. a wire bond, will inevitably deteriorate the ability of the device to resist electrostatic discharges (ESD).
In order to achieve a flat sensing surface, it is possible to provide a layer of top coating which is sufficiently thick so that the protruding wire bond is covered, thereby forming a flat outer surface. However, a thicker coating result in a weaker capacitive coupling between a finger placed on the surface and a sensing element located under the coating, which leads to a reduced accuracy of the sensing device.
US2011/0254108 discloses a fingerprint sensing device where the aforementioned problem is addressed by providing a protective plate having dielectric properties enhancing the capacitive coupling between a finger on the surface of the plate and a sensing element located underneath the protective plate.
Even so, it is desirable to be able to provide a top layer of a fingerprint sensing device having a thickness which is determined by the desired capacitive coupling properties of the sensing device, and not by the geometry of the electrical connection between a sensing chip and a substrate.